1. Field of the Invention
This invention relates to a method of, and an apparatus for, producing a near field optical head.
2. Description of the Related Art
A near field optical device has been presently utilized as a near field optical head of an information recording/reproducing apparatus or as a probe for optically observing a sample, and studies of its utilization have also been conducted.
A capacity of an information recording/reproducing apparatus using light has been increased while it scale has been decreased, and a higher density of a recording capacity has been required. To satisfy the requirements, studies have been made by use of blue violet semiconductor laser. However, these technologies may improve only several times the present recording density because of the diffraction limit of light. In contrast, an information recording/reproducing method utilizing near field light is expected as a technology handling optical information in a fine region exceeding the diffraction limit of light.
This technology utilizes near field light generated in the proximity of an optical aperture having a size smaller than a wavelength of light and formed in a near field optical head as a near field optical device. To reproduce optical information, it is possible to employ a method (illumination mode) that irradiates near field light generated from the fine aperture to a surface of a recording medium, and detects scattered light converted by an interaction with the surface of the recording medium on which a fine concavo-convexity recording information and optical constants such as a refractive index change, through a light reception device arranged separately. Near field light locally existing on the recording medium can be utilized, too. A method (collection mode method) that irradiates light to the surface of the recording medium and converts near field light locally existing at fine marks on the recording medium to scattered light through the interaction with the fine aperture and the fine protuberance can be employed as a reproduction method of optical information. In consequence, it becomes possible to handle optical information in a region below the wavelength of light that has been the limit in the optical system according to the prior art. To record information, a method that irradiates near field light generated from the fine aperture to the surface of the recording medium and changes the shape of the fine region on the medium (heat mode recording) and a method that changes a refractive index or a transmittance of the fine region (photon mode recording) can be used. A higher density than that of the information recording/reproducing apparatus of the prior art can be accomplished by using the near field optical head having the optical fine aperture or fine protuberance exceeding the diffraction limit of light.
Generally, the construction of the recording/reproducing apparatus utilizing near field light is substantially the same as the construction of magnetic disk apparatuses with the exception that a near field optical head is used in place of a magnetic head. The near field optical head fitted to a distal end of a suspension arm and having an optical fine aperture or fine protuberance is allowed to float to a predetermined height by use of a flying head technology using an air bearing to gain access to an arbitrary data mark existing on a disk. To let the near field optical head follow the disk rotating at a high speed, a flexure function is provided for stabilizing the posture in such a fashion as to correspond to swelling of the disk (refer, for example to Patent Reference 1).
Generally, optical resolution of a recording/reproducing apparatus utilizing near field light greatly depends on a proximity distance. Therefore, in the near field optical head of such an apparatus, the optical fine aperture or fine protuberance is formed to a height substantially equal to that of an air bearing surface, and the optical fine aperture or fine protuberance is brought close to the medium by a distance approximate to the floating distance of the near field optical head to acquire high optical resolution (refer, for example, to Patent Reference 2).
FIG. 8 shows a section of a near field optical head having the optical fine aperture produced according to a prior art method. A protuberance 803 and an air bearing 802 are formed on a substrate 801, and a shielding film 804 is formed on them. A distal end of the protuberance 803 is exposed from the shielding film 804 to form an optical fine aperture 805. The substrate 801 uses a dielectric having a high transmittance in a visible ray range such as quartz glass or diamond, a dielectric having a high transmittance in an infrared ray range such as zinc selenium or silicon, or a material having a high transmittance in a ultraviolet ray range such as magnesium fluoride or calcium fluoride. The shielding film 804 uses a metal such as aluminum, chromium, gold, platinum, silver, copper, titanium, tungsten, nickel or cobalt, or its alloy. The thickness of the shielding film 804 varies with its material but is from dozens to hundreds of nm. The protuberance 803 and the air bearing 802 can be collectively formed through etching technology of the substrate 801. The shielding film can be formed by use of a sputtering method or a vacuum deposition method and the like. To partially remove the apex of the protuberance of the shielding film 804 when forming the optical fine aperture 805, a method that uses a focused ion beam (FIB) (refer, for example, to Patent Reference 2) or a method that pushes a hard flat sheet to the apex of the protuberance and allows the shielding film 804 to undergo plastic deformation (refer, for example, to Patent Reference 3) can be employed.
The near field optical head using the fine protuberance, too, has been produced by a method substantially the same as the methods described above with the exception that the shielding film and the aperture are not formed.                Patent Reference 1:        JP-A-2001-34981 (page 4, FIG. 1)        Patent Reference 2:        JP-A-11-265520 (pages 6 to 7, FIG. 10)        Patent Reference 3:        JP-B-21201 (pages 3 to 4, FIG. 5)        
However, the production method of the near field optical head described above collectively forms the protuberance and the air bearing by etching and may therefore seem efficient at first sight. However, because an etching rate of the protuberance is different from that of the air bearing when forming them, it is difficult to simultaneously process the protuberance and the air bearing and to form the distal end of the protuberance and the surface of the air bearing on substantially the same plane. The protuberance has a greater etching rate due to its three-dimensional shape than the air bearing. Therefore, when the air bearing attains a predetermined height, the height of the protuberance is lower than that of the air bearing. When the protuberance is used as the near field optical head, it is not possible to bring the aperture and the protuberance sufficiently close to the medium. In consequence, optical resolution drops.
When etching is conducted to form the protuberance and the air bearing in the prior art methods, sensing of the etching amount is not made during etching but the etching amount is controlled only through the time control, and is measured after etching is once stopped. However, precise control of the etching amount through time control is difficult due to the changes of a density distribution and a density of an etchant. On the other hand, repetition of the measurement of the etching amount by interrupting the etching process invites the increase of the process steps and is not suitable for mass-production of near field optical heads having high quality.
On the contrary, it may be possible to form the air bearing by an additional processing such as deposition of a film, but it is difficult to form the surface of the air bearing and the distal end of the protuberance on substantially the same plane. For, it is difficult to accurately control the film thickness when the film is deposited.
As to the formation of the aperture, the prior art methods such as the method that uses FIB and the method that pushes the flat sheet are not efficient and are not entirely effective for mass-producing the near field optical heads having high quality.